Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Goodwill [Line Items]
Total goodwill	$ 11,915.5	$ 11,475.6
Unites States [Member]
Goodwill [Line Items]
Total goodwill	6,726.5	6,379.3
United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	2,521.5	2,511.9
Canada [Member]
Goodwill [Line Items]
Total goodwill	606.8	623.7
Australia [Member]
Goodwill [Line Items]
Total goodwill	757.1	650.5
New Zealand [Member]
Goodwill [Line Items]
Total goodwill	206.2	213.8
Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	1,097.4	1,096.4
Brokerage [Member]
Goodwill [Line Items]
Total goodwill	11,559.7	11,217.8
Risk Management [Member]
Goodwill [Line Items]
Total goodwill	336.9	238.8
Corporate [Member]
Goodwill [Line Items]
Total goodwill	18.9	19.0
Operating Segments [Member] | Brokerage [Member]
Goodwill [Line Items]
Total goodwill	11,559.7	11,217.8
Operating Segments [Member] | Brokerage [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill	6,651.7	6,304.5
Operating Segments [Member] | Brokerage [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	2,503.1	2,493.4
Operating Segments [Member] | Brokerage [Member] | Canada [Member]
Goodwill [Line Items]
Total goodwill	606.8	623.7
Operating Segments [Member] | Brokerage [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill	522.6	514.6
Operating Segments [Member] | Brokerage [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill	197.0	204.2
Operating Segments [Member] | Brokerage [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	1,078.5	1,077.4
Operating Segments [Member] | Risk Management [Member]
Goodwill [Line Items]
Total goodwill	336.9	238.8
Operating Segments [Member] | Risk Management [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill	74.8	74.8
Operating Segments [Member] | Risk Management [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	18.4	18.5
Operating Segments [Member] | Risk Management [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill	234.5	135.9
Operating Segments [Member] | Risk Management [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill	9.2	9.6
Operating Segments [Member] | Corporate [Member]
Goodwill [Line Items]
Total goodwill	18.9	19.0
Operating Segments [Member] | Corporate [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	$ 18.9	$ 19.0